The subject matter of this Application is related in part to that of the following two U.S. patent applications: Ser. No. 09/418,052, filed Oct. 14, 1999; and Ser. No. 09/447,882, filed Nov. 23, 1999. Said applications relate generally to the problem of maintaining an optimal gas mixture in a gas discharge laser and are hereby incorporated by reference in their entireties.
The invention relates to a means and method of monitoring the concentration of F2 gas in a molecular fluorine laser. Knowledge of the F2 concentration facilitates the maintenance of an optimal gas mixture in the discharge chamber of the laser.
Gas discharge lasers are well known as valuable tools for many industrial applications. In such lasers a mixture of gases in a discharge chamber is excited by an electrical power source to generate laser radiation. The mixture of gases may commonly include a halogen gas. The gain and thus the output of the laser is related to the composition of the chamber gas mixture and to the purity of the mixture. When the laser is operating, there is a tendency for halogen gas present in the mixture to react with other materials in the discharge chamber. In turn these reactions cause a depletion of the halogen and a reduction in the gain of the laser.
On the other hand, in industrial applications there is a great desire to have precise control and simultaneous stabilization of many laser parameters over extended periods of operation, especially with regard to excimer laser applications in microphotolithography. The amount of xe2x80x9cup timexe2x80x9d of a laser, or time when the laser is in operation and being used for industrial application, is a key variable in operational cost considerations. It is desired to be able to successively adjust, sensitively control and carefully stabilize various laser parameters efficiently and simultaneously. The type and quality of the gas discharge affects many significant laser parameters such as output power, energy stability, efficiency, bandwidth, long and short axial beam profiles, temporal and spatial pulse width, and beam divergence. The quality of the gas discharge depends on such factors as the composition of the gas mixture in the discharge chamber, the quality of preionization used, properties of the discharge circuit, and profiles of the electrodes used. See R. S. Taylor, Appl. Phys. B41, 1-24 (1986). Decomposition and contamination of the gas mixture and the design of the gas exchange system (e.g., flow speed) also strongly determine the limits of achievable laser parameters. A fast gas exchange between electrodes may be realized by using a laser discharge chamber design including fast blower gas circulation. Cryogenic and electrostatic equipment and techniques may be used for additional gas purification. See German Patent No. 32 12 928.
Optimal gas mixtures for various gas discharge lasers are generally known. A partition of F2 and buffer gas having a ratio of partial pressures of 0.3-2.5 for F2 to 1000 for the buffer gas is thought to be substantially optimal for an F2 excimer laser. The buffer gas is typically Helium or Neon, or a mixture thereof. As noted above, when a gas discharge laser containing a halogen gas is operated, over time the gas mixture continuously degrades or xe2x80x9cagesxe2x80x9d because of chemical reactions between the halogen and other materials including metal dust. In the case of an F2 excimer laser this means that there is a consumption of F2 over time when the laser is operated. U.S. Pat. No. 4,977,573 to Bittenson et al., which is assigned to the same assignee as the present application, relates to this problem of halogen consumption and is incorporated herein by reference in its entirety.
In order to maintain the laser""s operating characteristics, from time to time it is necessary to replenish the halogen gas in some way so as to substantially restore the original partition of the gas mixture.
It is desirable to have suitable measuring tools that indicate when and to what extent the laser gas mixture is aged before problems appear. It is further desirable to avoid significant reductions in laser output performance, processing errors, and excessive laser downtime.
A mass spectrometer may be used for precise analysis of the composition of the gas mixture. See U.S. Pat. No. 5,090,020 to Bedwell. However, a mass spectrometer is an undesirably heavy and costly piece of equipment to incorporate into a continuously operating excimer or molecular laser system. Other ways of monitoring the status of a laser gas mixture include measuring a spectrum width or bandwidth of a laser emission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring a beam profile of the laser emission (see U.S. Pat. No. 5,642,374 to Wakabayashi et al.), and measuring other characteristics such as the width of the discharge or temporal pulse width of the output beam wherein a rough estimate of the status of the gas mixture may be made. See U.S. Pat. No. 5,440,578 to Sandstrom. Another known technique of measuring the age of the laser gas mixture is to count the total number of laser pulses from the most recent new fill of the discharge chamber. See U.S. Pat. No. 5,646,954 to Das et al. A number of techniques are known wherein the output beam energy or efficiency is monitored and steps are taken to maintain the output beam at an optimal energy. See U.S. Pat. Nos. 3,899,750 to Hochuli, U.S. Pat. No. 4,429,392 to Yoshida et al., and U.S. Pat. No. 4,977,573 to Bittenson et al. Similarly see Mieko Okiwa and Minora Obara, Applied Physics Letters S1, (13), Sep. 28, 1987, and I. G. Koprinkov, K. V. Stamenov, and K. A. Stankov, Applied Phys. B33, 23.5-2.38 (1984). These articles teach the application of F2-injections based on measurement of an applied high voltage. Again, such methods do not rely on any knowledge of the actual F2/F-concentration in the gas discharge chamber of the excimer laser.
Rare and halogen gas concentrations have also been maintained by using a complex series of chemical reactions to determine the gas mixture concentrations and replenish depleted gases as needed. See U.S. Pat. No. 4,740,982 to Hakuta et al.
It is desirable then to have an efficient, practical, and inexpensive technique for monitoring the gas mixture status without variations in other parameters affecting the analysis.
It is an object of the present invention to provide an efficient and practical means and method for monitoring the F2 concentration in an F2 excimer laser. The present invention monitors the F2 concentration by measuring the concentration of F atoms and more specifically by measuring the amount of red laser light emitted during discharge. The amount of red laser light emitted during discharge is a function of the concentration of F atoms because such red laser light is emitted by excited F atoms. There is also a relationship between the concentration of F atoms and the concentration of molecular F2 in the discharge chamber. Accordingly, it is possible to monitor the concentration of F2 gas by monitoring the amount of red laser light emitted during discharge.